Molecular Geometry

VSEPR

Molecules in 3DMolecules in 3D

We saw in our last class that to properly address covalent bonding we need to use a pictorial approach, so we turn to Lewis Structures.

These are excellent because they give us a great perspective on the electronic arrangement in a molecule.

There is a drawback to a basic Lewis diagram and that is we do not get a sense for how the molecule exits in 3D.

VSEPR TheoryVSEPR Theory

Valence Shell Electron Pair Repulsion Theory . . . Huh?!?!

Electrons take up space!

Electrons are all negatively charged, therefore they don’t want to be near other electrons.

Electron pairs will repel other electron pairs.

Molecules will orient themselves in 3 dimensions to minimize the interactions of all the electron pairs.

A lone pair takes up more space than a bonding pair (ie they push bonding pairs further away).

Possible 3D ShapesPossible 3D Shapes

# of groups connected to the central atom

# of lone pairs on the central atom

Shape Bond Angles

Example

- - Linear Diatomic HCl

2 0 Linear Triatomic 180 º CO2

2 2 Bent (angular) 104.5 º H2O

3 0 Trigonal Planar 120 º BF3

3 1 Trigonal Pyramidal 107 º NH3

4 0 Tetrahedral 109.5 º CH4, CCl4

5 0 Trigonal Bi-pyramidal

120 º, 90 º

PCl5

6 0 Octahedral 90 º PCl6-

LinearLinear

Bond angle of 180 º minimizes all e- - e- interactions, giving maximum separation between all substituents (things connected to) on the central atom.

TetrahedralTetrahedral

Bond angle of 109.5 º minimizes all e- - e- interactions, giving maximum separation between all substituents (things connected to) on the central atom.

The tetrahedral geometry is very stable and very strong.

DiamondDiamond

Tetrahedral Lattice of Carbon AtomsTetrahedral Lattice of Carbon Atoms

A Diamond is the among the “hardest” substances in the known universe. It gains this strength from its molecular geometry, each carbon has a maximum spacing in the tetrahedral lattice, minimizing any destabilizing interactions.

What about Silicon What about Silicon Dioxide?Dioxide?

Quartz (crystalline SiO2)

Sand (crushed crystalline SiO2)

We know it’s NOT linear triatomic, so what’s the deal?

Structure and Bonding in Silicon DioxideStructure and Bonding in Silicon Dioxide

Every Si centre is tetrahedral

Every O centre is bent

Remarkably strong when a crystalline solid (quartz)

Structure and Bonding in Silicon DioxideStructure and Bonding in Silicon Dioxide

SiOSiO22

Amorphous and Amorphous and EverywhereEverywhere

SiO2 is also common glass, it has a structure much different than Quartz. All of the bonds are the same; however typical glass “looks” like a liquid that has been flash frozen. It’s arrangement is irregular and fluid.

TetrahedralTetrahedral

Bond angle of 109.5 º minimizes all e- - e- interactions, giving maximum separation between all substituents (things connected to) on the central atom.

The tetrahedral geometry is very stable and very strong.

Trigonal Planar and PyramidalTrigonal Planar and Pyramidal

120 º bond angle between each F atom, maximum separation

107 º bond angle between each F atom, maximum separation

Derived from a tetrahedral arrangement, with the “top” atom replaced by a lone pair.

BENTBENT

Only 90 º between H and lone pair.

104.5 º between the 2 H atoms

Water V shaped or BENT Derived from a tetrahedral arrangement, with 2 atoms replaced by lone pairs.

Practice TimePractice TimeWhat are the shapes and bond angles in each of the molecules from yesterday?

HCN, N2H4, CO2, CO, NI3, SCl2, AsCl3, PCl3, CH4, NH4+, HCl, BH3, O3,

CCl4, SiCl4, SiO2, CH2Cl2, IF, AlCl3, CH2O, CH2S

# of groups connected to the central atom

# of lone pairs on the central atom

Shape Bond Angles

Example

- - Linear Diatomic HCl

2 0 Linear Triatomic 180 º CO2

2 2 Bent (angular) 104.5 º H2O

3 0 Trigonal Planar 120 º BF3

3 1 Trigonal Pyramidal

107 º NH3

4 0 Tetrahedral 109.5 º CH4, CCl4